Certain present borehole televiewer tools or sondes provide detailed acoustic sensing of the wall of the borehole through the use of sonic transmitters and receivers. Such sondes provide the travel times of an acoustic pulse and the relative amplitude of the reflected acoustic pulse. In addition, the sonde provides a relative reference of the sonde in the borehole with respect to the north pole. From this data, images of the borehole wall may be constructed based on either the acoustic reflectivity of the borehole wall or the distance from the televiewer sonde to the borehole wall. Such images typically reveal information regarding the texture and shape of the borehole wall, and features such as fractures and bedding planes which may intersect the borehole.
These reflection and travel time images are conventionally digitized and displayed on a digital CRT, where variation in color or gray scale correspond to changes in reflectivity or travel time. Unfortunately, the travel time image display dynamic range is often saturated by variations in the sonde-to-borehole wall distance brought about by slight eccentricity of the sonde within the borehole. Or stated another way, if the sonde is not in the center of the borehole, but rather closer to one wall, the differences in travel time caused by differences in shape of the borehole wall tend to be obscured by the differences caused by the location of the sonde with respect to the center of the borehole. This renders detection of small variations in borehole wall topography or ellipticity difficult to detect. Further, a detailed comparison between reflectance and travel time data is difficult when there is marked eccentricity. Also, the borehole ellipticity (which is related to in situ stress fields) is difficult to quantify from such an image.
It can, therefore, be appreciated that a method to determine the eccentricity of the sonde within the borehole and to correct the travel time data to compensate for such an eccentricity is highly desirable.